Optical discs are widely used as mediums for recording sounds, videos, document data, and the like. There have been developed various kinds of devices for recording or reproducing information to or from optical discs.
Such a device includes an optical pickup device as a main component. At a forefront of the device, the optical pickup device receives an optical signal (laser signal) from an optical disc, converts the optical signal into an electrical signal, and outputs the electrical signal. More specifically, the optical pickup device is a main component which receives a laser beam reflected on the optical disc, converts the laser beam into an electrical signal (current) through a light-receiving element (photodiode), converts the electrical signal into a voltage through a gain resistor provided in a circuit connected to the light-receiving element, and outputs the voltage to an LSI provided downstream.
In general, the current obtained by conversion through the light-receiving element is very weak. It is therefore necessary to amplify the current through an amplifier circuit (light-receiving amplifier element). FIG. 7 illustrates an equivalent circuit of a generally used light-receiving amplifier element. FIG. 8 illustrates a shape and a position of a light-receiving section of the light-receiving element.
In the generally used amplifier element, as shown in FIG. 7, an electrical signal Isc obtained by conversion through a light-receiving element 201 is amplified by two serially connected amplifier circuits (a first-stage amplifier circuit A11 and a following-stage amplifier circuit A12).
As shown in FIG. 8, the light-receiving element 201 includes main light-receiving sections 202 (A through D) and sub light-receiving sections 203 (E through H). The main light-receiving sections 202 are four regions provided at the center of the light-receiving element 201. The sub light-receiving sections 203 are provided on the right and left of the main light-receiving sections 202. In the optical pickup device, an electrical signal obtained by conversion through the main light-receiving sections 202 is used for performing a focus adjustment and reproducing a data signal, and an electrical signal obtained by conversion through the sub light-receiving sections 203 is used for performing a tracking adjustment.
During operation of the optical pickup device, each of the main light-receiving sections 202 and of the sub light-receiving sections 203 is irradiated with the laser beam reflected on the optical disc. Therefore, an output terminal of the light-receiving element 201 is active. That is, as shown in FIG. 7, the laser signal is converted into the electrical signal Isc through the light-receiving element 201, the electrical signal Isc is subjected to current-to-voltage conversion and amplification through the first-stage amplifier circuit A11, and then to voltage amplification through the following-stage amplifier circuit A12, and is outputted from an output terminal 101.
In order to stabilize operation of the circuit as a whole, it is necessary that the light-receiving amplifier element including the serially connected amplifier circuits be free from a noise or oscillations on power supply lines, and other problems. For this purpose, in the light-receiving amplifier element of FIG. 7, the amplifier circuits (A11 and A12) and another circuit are provided with metal wires 103 through 105, which are separate power supply lines for supplying power from a power source pad 102. For example, the following Patent Publication 1 describes an arrangement of supplying power through separate power supply lines to a plurality of serially connected amplifier circuits. FIG. 9 illustrates a schematic diagram of the power supply lines (metal wires) in the vicinity of the power source pad 102.
[Patent Publication 1]
Japanese Publication for Unexamined Patent Publication, Tokukaihei 2003-37446 (publication date: Feb. 7, 2003)
In the example of FIG. 9, power supply lines connecting the power source pad 102 with each circuit are provided independently, so as to reduce the noise on the power supply lines. Specifically, the metal wire 103 is provided for supplying power to the first-stage amplifier circuit; the metal wire 104 is provided for supplying power to the following-stage amplifier circuit, and the metal wire 105 is provided for supplying power to the another circuit.
In a multi-stage amplifier circuit including a plurality of serially connected amplifier circuits as described above, a coupling noise is generated by a rapid voltage fluctuation at an output terminal of the first-stage amplifier circuit. Therefore, according to the foregoing circuit arrangement, not only an amplified signal but also the voltage fluctuation caused in the first-stage amplifier circuit is supplied to the following-stage amplifier circuit through the power source pad 102. As a result, circuit operation is destabilized.
The coupling noise is generated for various reasons. A salient reason is that a metal layer and an insulating layer, which are laminated in the integrated circuit, are not jointed properly. In forming capacitors and resistors in an integrated state on a chip, a large number of metal layers and insulating layers are laminated. In this case, if there is a defective portion where a metal layer and an insulating layer are not jointed properly, the defective portion generates a noise (hereinafter “junction noise”).
The integrated circuit is also affected electrically by a noise generated outside the integrated circuit (noise generated by life environment; hereinafter “external noise”)
The term “coupling noise” is a generic term of the junction noise and the external noise.
It is because the coupling noise destabilizes the power sources (Vcc, Vref) that the rapid voltage fluctuation is caused at the output terminal of the first-stage amplifier circuit as described above. The rapid voltage fluctuation at the output terminal further generates the coupling noise, thereby creating a vicious circle.